In general, an image pickup sensor, such as a CMOS image sensor, has a plurality of image pickup elements, each of which defines a unit pixel (corresponding to one pixel). In recent years, in an image pickup apparatus, such as a digital camera or a video camera, an image pickup sensor, such as a CCD (Charge Coupled Device) or a CMOS (Complementary Metal Oxide Semiconductor) image sensor is used.
Further, as the pixel size reduction and the like due to a higher pixel count proceeds, there has been an increasing demand for ideas and devices for expanding the dynamic range of an image pickup sensor with respect to an optical input (i.e. optical signal) thereto.
For example, in PTL 1 to PTL 3, there have been proposed techniques in which a charge holding portion other than a floating diffusion portion is provided in each image pickup element that defines a unit pixel, and the dynamic range is expanded by the charge holding portion.
As mentioned above, in PTL 1 to PTL 3, each image pickup element has a structure provided with the charge holding portion different from the floating diffusion portion. Particularly, in PTL 2 and PTL 3, electric charge overflowing from a photo diode during charge accumulation time is accumulated in the charge holding portion via an overflow gate, and the dynamic range is ensured thereby.
Now, FIG. 13 is a diagram showing an example of a circuit forming a pixel 60 used for a CMOS image sensor.
For example, the CMOS image sensor has a plurality of pixels 60, and these pixels 60 are arranged in a matrix form in the CMOS image sensor.
The pixel 60 illustrated in the figure has a photo diode (hereinafter referred to as “PD”) 61. The PD 61 receives an optical input (i.e. optical image) subjected to image formation by a photographic lens (not shown) to thereby generate and accumulate electric charge. A first transfer transistor 62 is a transistor used as a first transfer switch and is implemented by a MOS transistor.
A floating diffusion portion (hereinafter referred to as “FD”) 64 holds electric charge accumulated in the PD 61 as voltage. A reset transistor 63 resets the electric potential of the FD 64. An amplifier transistor 65 is connected as a source follower to the FD 64, and a row selection transistor 66 is used for selecting a row of pixels in the CMOS image sensor. The output voltage is output to a vertical output line 67 via the row selection transistor 66.
A charge holding portion (hereinafter referred to as “MEM”) 68 is arranged between the PD 61 and the FD 64. Electric charge is transferred from the PD 61 to the MEM 68 via the first transfer transistor 62. The electric charge accumulated in the MEM 68 is transferred to the FD 64 via a second transfer transistor 69 used as a second transfer switch.
FIG. 14 is a diagram illustrating potentials in the above-mentioned pixel 60. In FIG. 14, a barrier TX1 defined by the first transfer transistor 62 exists between the PD 61 and the MEM 68. Further, a barrier TX2 defined by the second transfer transistor 69 exists between the MEM 68 and the FD 64.
The barrier TX1 between the PD 61 and the MEM 68 is configured such that it is higher than a maximum electric charge which can be stored (hereinafter referred to as “charge storage capacity”) and at the same time, an overflowing electric charge from the PD 61 flows out in a direction toward the MEM 68. When electric charge generated by an optical input exceeds the charge storage capacity of the PD 61, electric charge flows over the barrier TX1 and is stored in the MEM 68.
It should be noted that depending on the capacity configuration of the MEM 68, it may be configured such that during charge accumulation, electric charge accumulated in the PD 61 is always transferred to the MEM 68 by activating the first transfer transistor 62.
By the way, not only in the PD 61 and the FD 64 but also in the MEM 68, noise occurs which is caused by dark current that increases as time elapses. Therefore, when the dynamic range is expanded by accumulating electric charge generated by the optical input in the PD 61 and the MEM 68, during charge accumulation, it is impossible to eliminate a noise component caused by dark current in the MEM 68.
As a consequence, when a comparison is made between a pixel without the MEM 68 and the pixel 60 with the MEM 68, even when the conditions, such as the areas of the PD 61 and the FD 64, of the two are the same, the pixel 60 with the MEM 68 delivers a larger noise to the CMOS image sensor, due to a noise component caused by dark current in the MEM 68.
Further, in high sensitivity photographing or the like in which amplification is performed at a later stage than the pixel 60 or in a later stage than the CMOS image sensor, an insufficient dynamic range of the pixel 60 is less liable to occur since the optical input is inherently small. For this reason, it is not very necessary to expand the dynamic range.
FIG. 15 is a diagram showing an example of difference in the amount of electric charge to be accumulated in the PD 61 occurring depending on the configuration of photographic recording sensitivity when obtaining an image by the same exposure.
The photographic recording sensitivity (ISO sensitivity) in an image pickup apparatus is determined by “electric charge accumulated in the PD 61×circuit gain in a later stage”. Therefore, when photographing is performed with the same exposure with respect to a luminance of an object, ISO 100=100% of electric charge amount in the PD 61×a circuit gain of 1, and ISO 200=50% of electric charge amount in the PD 61×a circuit gain of 2. Therefore, as the ISO sensitivity becomes higher, the margin to the capacity of the PD 61 increases (i.e. electric charge is less likely to overflow).
Further, since the optical input required by the PD 61 becomes smaller, an insufficient dynamic range of the pixel 60 is less liable to occur. On the other hand, if the dynamic range is expanded, the noise component caused by dark current dependent on time is generated as much as in low sensitivity photographing, and the noise component due to dark current is amplified since the amplification is performed in the later stage, which tends to degrade image quality liable to occur.
For example, in the case of ISO 100, the circuit gain in the later stage is 1, so that the dark current generated in the pixel 60 has an influence corresponding to the generated amount thereof. However, in the case of ISO 200, the circuit gain in the later stage is doubled, so that the dark current has an influence corresponding to twice as much as the generated amount thereof.